rc diff: make it a module

We want to move git-diff-goto-source from rc/tools/git.kak
to rc/filetype/diff.kak (or should we could create
rc/tools/diff.kak?). Either way, create the diff module so we can
formalize this dependency.

Currently this module only provides highlighters, so require it
wherever we reference them.

Keep the diff-select-{file,hunk} commands outside the module because
people might already use them in git buffers.

rc/filetype/diff.kak

@@ -2,6 +2,12 @@ hook global BufCreate .*\.(diff|patch) %{
     set-option buffer filetype diff
 }
 
+hook global WinSetOption filetype=diff %{
+    require-module diff
+}
+
+provide-module diff %§
+
 add-highlighter shared/diff group
 add-highlighter shared/diff/ regex "^\+[^\n]*\n" 0:green,default
 add-highlighter shared/diff/ regex "^-[^\n]*\n" 0:red,default
@@ -12,6 +18,8 @@ hook -group diff-highlight global WinSetOption filetype=diff %{
     hook -once -always window WinSetOption filetype=.* %{ remove-highlighter window/diff }
 }
 
+§
+
 define-command \
     -docstring %{diff-select-file: Select surrounding patch file} \
     -params 0 \

rc/filetype/git.kak

@@ -35,6 +35,7 @@ hook -group git-rebase-highlight global WinSetOption filetype=git-rebase %{
 
 
 provide-module git-commit %{
+require-module diff
 add-highlighter shared/git-commit regions
 add-highlighter shared/git-commit/diff region '^diff --git' '^(?=diff --git)' ref diff # highlight potential diffs from the -v option
 add-highlighter shared/git-commit/comments region ^# $ group

rc/tools/git.kak

@@ -2,6 +2,7 @@ declare-option -docstring "name of the client in which documentation is to be di
     str docsclient
 
 hook -group git-log-highlight global WinSetOption filetype=git-log %{
+    require-module diff
     add-highlighter window/git-log group
     add-highlighter window/git-log/ regex '^([*|\\ /_.-])*' 0:keyword
     add-highlighter window/git-log/ regex '^( ?[*|\\ /_.-])*\h{,3}(commit )?(\b[0-9a-f]{4,40}\b)' 2:keyword 3:comment
@@ -12,6 +13,7 @@ hook -group git-log-highlight global WinSetOption filetype=git-log %{
 }
 
 hook -group git-status-highlight global WinSetOption filetype=git-status %{
+    require-module diff
     add-highlighter window/git-status group
     add-highlighter window/git-status/ regex '^## ' 0:comment
     add-highlighter window/git-status/ regex '^## (\S*[^\s\.@])' 1:green